1. Field of the Invention
The present invention relates generally to a high-efficiency Alternating Current (AC)-driven Light-Emitting Diode (LED) module and, more particularly, to a high-efficiency AC-driven LED module that improves power and current differences attributable to variations in input voltage and an undesirable power factor, that is, the most important problems of conventional AC LEDs, using a method of rectifying commercial voltage using a bridge diode, sequentially controlling the sections of an appropriate (maximum commercial voltage*√2/Diode VF) number of LED chips connected in series, and driving them while increasing current, so that regulations (equal to or higher than PF 0.9) in respective countries can be met and also the power efficiency of an LED lamp (to a value equal to or higher than about 95%) can be maximized by minimizing control loss.
2. Description of the Related Art
Light-Emitting Diodes (LEDs) are current-driven devices, and can operate only when constant current is stably supplied thereto.
Of these LEDs, small-sized LEDs may be controlled using the method shown in FIG. 1.
For example, FIG. 1 illustrates one method for controlling LED lamps. This method has the disadvantage of the manufacturing cost of a related system being expensive because LEDs are lit using a Switched-Mode Power Supply (SMPS), that is, a conventional DC voltage conversion device, and a constant current driver.
In particular, it is necessary to solve various problems with the control method, that is, the problem of the short life span of LEDs attributable to the short life span (MTBF: about 10,000 HR-20,000 HR) of electrolytic capacitors C1, C3, C4 and C5, the problem of electromagnetic interference attributable to the SMPS switching method, the problem of heat dissipation attributable to the assembly structure, and a low energy conversion efficiency of about 80%.
In order to mitigate the above problems, some LED manufacturers developed the AC LEDs shown in FIG. 2. Because of the fundamental characteristic of LEDs that they are sensitive to excessive voltage (excessive current), such AC LEDs are being sold and manufactured at working voltages at regular intervals of 10 V.
Furthermore, this issue is related to the most important problem. Although AC LEDs themselves can comply with the power factor regulations (being equal to or higher than 0.85 in the case of LEDs equal to or lower than 5 W), the LED lamps (bulbs, down-lights, tubes, or flat or street lights) using such AC LEDs cannot comply with the power factor regulations (being equal to or higher than 0.9) in terms of the input power of a lamp, as shown in FIG. 3, and therefore it is difficult to obtain lamp standard certifications in respective countries.
Furthermore, the efficiency thereof is poor because the AC LEDs are composed only of passive elements, such as bridge diodes and resistors, and the brightness of an LED lamp is not constant but abruptly changes because the current is not actively controlled in connection with variations in input voltage and therefore input power and current abruptly decrease or increase in the range of variations in voltage.
The AC LEDs are problematic in that it is difficult to satisfy the respective standards of countries, the life span and optical flux of the AC LEDs may be degraded by heat, and the reliability of the AC LEDs may be considerably decreased.
Although recently an AC-driven LED driver capable of mitigating the power factor problem of AC LEDs has been developed, a high loss occurs in active elements in the higher interval of a rated voltage range due to a relative decrease in efficiency in a constant current controller in terms of the VF characteristics of the LEDs, thereby degrading the efficiency of the LEDs.